1. Field of the Disclosure
The instant disclosure relates to a radio frequency (RF) power amplifier; in particular, to a RF power amplifier with temperature compensation.
2. Description of Related Art
Among hand-held wireless communication products, main direct current (DC) power consumption comes from a RF power amplifier. Therefore, to keep a high linearity of the RF power amplifier instead of distorting an amplified signal, and to maintain a high efficiency to support a long time of communication has always been a focus in designing the RF power amplifier. In particular, there is an obvious characteristic of a time-varying wave packet when a broadly used digital modulation technique of orthogonal frequency-division multiplexing (OFDM) is adopted in a wireless communication system, and a constant of peak to average power ratio (PAPR) of which is way higher than that of the current wireless communication system; in other words, the variation of the wave packet with time is more dramatic, and thus a demand of the linearity of the RF power amplifier is higher.
Referring to FIG. 1, the RF power amplifier 100 in prior arts includes a transistor Q1, resistors R1′ and R2′, and a transistor Q2, wherein the transistor Q1 is a depletion-type field effect transistor (FET), and the transistor Q2 is a bipolar junction transistor (BJT). A drain of the transistor Q1 receives a reference voltage VREF′; a terminal of the resistor R1′ is coupled to a source of the transistor Q1; another terminal of the resistor R1′ is coupled to a gate of the transistor Q1; a terminal of the resistor R2′ is coupled to another terminal of the resistor R1′; another terminal of the resistor R2′ is coupled to a base of the transistor Q2; an emitter of the transistor Q2 is coupled to a ground voltage GND, a collector of the transistor Q2 is coupled to a system voltage VCC′.
In the RF power amplifier 100 in the prior arts, the transistor Q1 outputs a current ID′ with a fixed value, and the current ID′ is equal to an input current IB′ (base current) of the transistor Q2. Since a current gain (B) of the transistor Q2 is with a negative temperature coefficient, and the current ID′ is a current with a close-to-zero temperature coefficient, an output current IC′ of the transistor Q2 is a current with a negative temperature coefficient and varies with a change of a surrounding temperature.
Referring to FIGS. 2A-2C, FIGS. 2A-2C are simulation waveforms corresponding to FIG. 1, wherein an abscissa in every drawings represents a temperature, and a range of the temperature is set from −40□ to +90□. In the FIG. 2A, an ordinate represents the current ID′, and the current ID′ as shown in FIG. 2A varies with the change of the surrounding temperature, and a value of the bias current ID′ is substantially equal to the fixed value. In FIG. 2B, an ordinate represents a current gain, and a value of the current gain decreases with an increasing temperature. In FIG. 2C, an ordinate represents the output current IC′, and the output current IC′ decreases with an increasing temperature, and further greatly affects an output power of the RF power amplifier 100 in the prior arts, which may fail to achieve a demand of the RF power amplifier operated in a high/low surrounding temperature in a current communication system.